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Thermodynamics

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Optical Instruments

The magnifying power of a telescope or a microscope depends on the focal length of the lenses used. By a proper choice of the lenses, it is possible to increase the size of the image, i.e. the image subtends a large angle at the eye. But, it must be remembered that increase in the size of the image, beyond a certain limit does not necessarily mean gain in detail. This is the case even if the lenses are free from all aberrations, chromatic and monochromatic. There is always a limit to the useful magnification of an optical instrument. This is due to the fact that for a wave surface the laws of geometrical optics do not hold good. In the preceding articles concerning diffraction of light, it has been shown that the image of a point source is not a point but it is a diffraction pattern. With a circular aperture kept in the path of incident light, the diffraction pattern of a point source of light consists of a central bright disc surrounded by alternately dark and bright diffraction rings.

If the lens diameter or the size of the aperture is large, the diffraction pattern of a point source of light is small. If there are two nearby point sources, the diffraction discs of the two patterns may overlap and the two images may not be distinguished. An optical instrument like a telescope or a microscope is said to have resolved the two point sources when the two diffraction patterns are well separated from one another or when the diffraction patterns are small so that in both the cases, the two images are seen as separate ones. The ability of an optical instrument, expressed in numerical measure, to resolve the images of two nearby points is termed as its resolving power.

In the case of a prism or a grating spectrograph, the term resolving power is referred to the ability of the prism or grating to resolve two nearby spectral lines so that the two lines can be viewed or photographed as separate lines.